Aqueous slurries of polyvinyl alcohol (PVA) polymer are useful in the adhesives industry as well as in the paper industry. To prepare aqueous slurries having a high-concentration of PVA polymer powder (high-concentration PVA slurries) it is necessary to disperse PVA polymer in water.
However, dispersing PVA polymer in water can be problematical. Aqueous PVA mixtures can exhibit strong non-Newtonian behavior (exhibiting Bingham-plastic, shear thinning, and time dependent thixotropic properties), and can require extensive agitation when viscosity is high. PVA polymer in water can form gels, or gel-like mixtures. This is particularly true for high-concentration aqueous PVA mixtures wherein the concentration of PVA in water is more than about 14% PVA by weight, based on the total weight of the PVA/water mixture. PVA/water xe2x80x9cgelsxe2x80x9d can be difficult to use in a manufacturing process because such PVA mixtures can be difficult, if not impossible, to agitate due to high viscosity.
Using conventional mixing techniques can be ineffective in preventing the formation of PVA/water gels. For example, it has been observed that turbine-type impellers do not provide adequate mixing near the surface and along tank circumferences of PVA water mixtures. As a result of weak agitation near the surface a local build-up in viscosity in that region can occur, yielding gels near or at the surface of PVA/water slurries. Also, it can be conventional to use baffles to disrupt laminar flow in a mixing process and reduce or eliminate a vortex formed by the spinning action of an impeller. However, baffles are not suitable for use in the practice of the present invention. In fact, it is surprising that baffles can exacerbate gel formation. Without being held to theory, one explanation of this surprising result is that the baffles act to xe2x80x9chold upxe2x80x9d PVA in the area of the baffles, thus causing a local viscosity increase in that region. Over time, the region high in viscosity can expand and cause the entire mixture to xe2x80x9cset upxe2x80x9d (that is, become gel-like).
It is desirable to prepare an aqueous mixture of polyvinyl alcohol having at least 14% by weight of PVA by a process whereby the viscosity of the mixture does not approximate that of a gel at any time during the process.
In one aspect, the present invention is a process for preparing an aqueous polyvinyl alcohol (PVA) mixture having at least about 14%, by weight, of PVA comprising the steps: (a) metering water (1) and PVA (2) into a mixing vessel that contains water and is equipped with an impeller for mixing the contents of the vessel, wherein the metered PVA is at least about 14%, by weight, of the total weight of (1) and (2), and wherein the impeller is rotated at a speed of at least about 60 rpm before the PVA is metered into the vessel; (b) increasing the speed of the impeller during the process of metering (1) and (2) into the vessel; (c) optionally recirculating a portion of the mixed contents of the vessel back into the vessel such that the recirculated mixture is injected into the vessel at a speed and at a location that eliminates the vortex created by the impeller rotational action; and (d) continuing the addition of (1) and (2) as in step (a) until the mixture has a PVA content of at least about 14%, by weight of the mixture.
In another aspect, the present invention is a process for preparing an aqueous polyvinyl alcohol (PVA) mixture having at least about 14%, by weight, of PVA comprising the steps: (a) metering water (1) and PVA (2) into a mixing vessel that contains water and is equipped with an impeller for mixing the contents of the vessel, wherein the metered PVA is at least about 14%, by weight, of the total weight of (1) and (2), and wherein the impeller is rotated at a speed of at least about 60 rpm before the PVA is metered into the vessel; (b) increasing the speed of the impeller during the process of metering (1) and (2) into the vessel; (c) optionally recirculating a portion of the mixed contents of the vessel back into the vessel such that the recirculated mixture is injected into the vessel at a speed and at a location that eliminates the vortex created by the impeller rotational action; and (d) continuing the addition of (1) and (2) as in step (a) until the mixture has a PVA content of at least about 14%, by weight of the mixture, wherein the impeller creates counter-flow mixing pattern in the mixture.
In still another aspect, the present invention is a process for preparing an aqueous polyvinyl alcohol (PVA) mixture having at least about 14%, by weight, of PVA comprising the steps: (a) metering water (1) and PVA (2) into a mixing vessel that contains water and is equipped with an impeller for mixing the contents of the vessel, wherein the metered PVA is at least about 14%, by weight, of the total weight of (1) and (2), and wherein the impeller is rotated at a speed of at least about 60 rpm before the PVA is metered into the vessel; (b) increasing the speed of the impeller during the process of metering (1) and (2) into the vessel; (c) optionally recirculating a portion of the mixed contents of the vessel back into the vessel such that the recirculated mixture is injected into the vessel at a speed and at a location that eliminates the vortex created by the impeller rotational action; and (d) continuing the addition of (1) and (2) as in step (a) until the mixture has a PVA content of at least about 14%, by weight of the mixture, wherein the vessel does not include baffles.
In another aspect, the present invention is an aqueous PVA mixture having at least about 14%, by weight, of PVA based on the total weight of the mixture made by the process comprising the steps: (a) metering water (1) and PVA (2) into a mixing vessel that contains water and is equipped with an impeller for mixing the contents of the vessel, wherein the metered PVA is at least about 14%, by weight, of the total weight of (1) and (2), and wherein the impeller is rotated at a speed of at least about 60 rpm before the PVA is metered into the vessel; (b) increasing the speed of the impeller during the process of metering (1) and (2) into the vessel; (c) optionally recirculating a portion of the mixed contents of the vessel back into the vessel such that the recirculated mixture is injected into the vessel at a speed and at a location that eliminates the vortex created by the impeller rotational action; and (d) continuing the addition of (1) and (2) as in step (a) until the mixture has a PVA content of at least about 14%, by weight of the mixture.